1. Technical Background:
The present invention relates generally to intravascular medical devices, and more particularly, to a medical guidewire for use with magnetic resonance systems. Such guidewires may be used in medical procedures for both diagnostic and interventional purposes.
2. Discussion:
Guidewires are used in a wide variety of medical procedures, most often in conjunction with one or more other medical devices, including catheters. Such a catheter may be any of various types, such as angiography or angioplasty, but should in any event have a tubular lumen or other guiding means through which the guidewire can be advanced or withdrawn.
Structurally, guidewires are often long, thin metal wires that generally taper from one diameter at a proximal end which remains outside the body of the patient, to a smaller diameter at the opposite distal end. Specifically, vascular guidewires are often more than five feet long and have a maximum outer diameter of approximately 0.038 inches. The diameter of the core wire is generally ground down precisely in a series of alternating tapering portions and constant diameter sections, to develop a selectively engineered flexibility profile along the length of the guidewire.
The guidewire distal tip is usually very flexible, both to avoid vascular trauma and so that it can be selectively bent and twisted to advance it along a desired vascular path. Guidewires a redesigned to resist this twisting force or torsion, so that as the guidewire proximal end is twisted or rotated, the distal tip tends to rotate through about the same angle. In addition, a floppy spring is often affixed to the extreme distal tip of the guidewire for flexibility.
A good example of a current guidewire is described in the commonly assigned U.S. Pat. No. 4,846,186, issued to Box et al. on Jul. 11, 1989, which is incorporated in this disclosure by reference. The Box patent shows a guidewire suitable for both diagnostic and therapeutic or interventional procedures, having a Teflon coating from the proximal end along a majority of its length. The core wire tapers in steps to a distal portion that is flattened and surrounded by a flexible spring, which is brazed to the extreme distal end of the core wire to form a rounded tip.
As the body of the patient is of course opaque, physicians commonly use fluoroscopy or X-ray video cameras to track the position of the guidewire and to construct real-time images of the patient""s vasculature. The visibility and brightness of selected portions of the guidewire is a relatively important feature, as described in the commonly assigned U.S. Pat. No. 5,259,393, issued to Corso, Jr. et al. on Nov. 9, 1993, and U.S. Pat. No. 5,267,574, issued to Viera et al. on Dec. 7, 1993. Both of these patents are incorporated in this disclosure by reference. In the Corso patent, the flexible spring at the guidewire distal tip is arranged to selectively control its brightness on an X-ray fluoroscope, or radiopacity. Likewise, the Viera patent discloses a plastic sleeve shrunk around an intermediate section of the guidewire, and several radiopaque marker bands.
In contrast to fluoroscopy, another method of visualizing the patient is magnetic resonance imaging, referred to as MRI. Other medical fields, such as neurology, often use procedures which are performed under MRI instead of X-ray fluoroscopy. Accordingly, it is also desirable to image the anatomy and to track the position of intravascular devices, including catheters and guidewires, using magnetic resonance (MR) systems.
For these applications, it is desirable to make guidewires usable and compatible with MRI techniques. However, a metal guidewire may be too visible under MR, brightly washing out the screen and obscuring important features. This halo phenomenon is called an xe2x80x9cartifact,xe2x80x9d and renders the image useless. Another issue with the use of a metal guidewire under MR is the induction of eddy currents in the metal, caused by distortion of the magnetic field. These eddy currents can generate heat and may increase the local temperature of the surrounding tissue and body fluids, thus possibly damaging the tissue or causing the blood to coagulate.
It is an object of the present invention to provide a guidewire having the desired physical features, including torsion and flexibility, while also avoiding the creation of undesirable artifacts in the MR image or the generation of heat.
The present invention provides a guidewire compatible for use with magnetic resonance systems, made from a non-metallic material with a high specific electric, impedance. Accordingly, this material will resist any electrical eddy currents in the guidewire from being generated by variations in the high-frequency field. An acceptable class of materials is glass, which are all electrical insulators. A guidewire having a major portion constructed of a glass material should therefore have the advantages of not disturbing the MR field and images, as well as resisting the generation of heat.
These and various other objects, advantages and features of the invention will become apparent from the following description and claims, when considered in conjunction with the appended drawings.